1. Field of the Invention
The present invention generally relates to an automatic control system for controlling a chemical process in order to maximize efficiency. More particularly, the invention relates to an automatic control system for controlling the operation of a phosphoric acid recovery plant in order to maintain constant efficiency.
Phosphoric acid (H.sub.3 PO.sub.4) is an important intermediate chemical product. It is used primarily by the fertilizer industry, but also is useful in a number of other areas such as in detergents, water treatment, and food products.
2. Description of Related Art
The most common method for producing phosphoric acid is known in the art as the "wet process." Using the wet process, phosphate rock, which comprises calcium and phosphorus, typically in the form Ca.sub.3 PO.sub.4, together with a number of impurities, is mined, beneficiated, and comminuted. Typically, ball mills or rod mills are used to obtain the desired size distribution. Grinding can be carried out by a wet or dry process; wet processing is typical. The ground rock then is fed, typically as an aqueous slurry, to a reactor, or "attack tank", and reacted with sulfuric acid. The process is represented by the following equation: EQU Ca.sub.3 (PO.sub.4).sub.2 +3 H.sub.2 SO.sub.4 +6 H.sub.2 O.fwdarw.2 H.sub.3 PO.sub.4 +3 CaSO.sub.4 .multidot.2 H.sub.2 O (A)
The phosphoric acid thus produced remains in aqueous solution; calcium sulfate, commonly referred to as gypsum, is a crystalline solid under reaction conditions. A simplified flow diagram of a wet process phosacid plant, including sulfuric acid manufacture, is illustrated in FIG. 1.
Although phosphoric acid is represented as H.sub.3 PO.sub.4, plant output most often is measured in tons of P.sub.2 O.sub.5, or phosphorous pentoxide. Approximately 4 tons of 30% phosphoric acid aqueous solution is equivalent to about 1 ton of P.sub.2 O.sub.5. It also is common in the trade to use the terms "phosphorous pentoxide" and "phosphoric acid" interchangeably when referring to plant output. "Phosacid" is a short form for phosphoric acid, typically in aqueous solution.
The operator of a phosphoric acid plant typically seeks to maximize the production of phosphoric acid from the limited amount of phosphate rock, thus minimizing any potential losses of phosphoric acid. To minimize cost, the operator must maximize overall plant efficiency. An important factor influencing the efficiency of a phosphoric acid plant is the amount of free or excess sulfate (SO.sub.4.sup.=, measured as weight percent of excess sulfate) in the reactor or attack tank. The amount of excess sulfate constitutes the amount of sulfate not reacted with calcium or in the form of CaSO.sub.4. On this basis, "excess sulfate" may have either a positive or a negative value (positive representing an excess of sulfate, and negative representing a deficiency of sulfate).
The reaction shown in Equation A produces two chemical products: phosphoric acid and calcium sulfate crystals (gypsum--a co-product). A properly operating phosphoric acid plant results in high P.sub.2 O.sub.5 recovery from the attack tank slurry and the formation of calcium sulfate crystals that contain little unreacted phosphate and have a shape, size, and porosity affording easy separation from the liquid by filtration. Level of excess sulfate is one of the most important factors governing crystallization quality.
There are primarily three types of P.sub.2 O.sub.5 losses that may occur in a phosphoric acid process. First are lattice losses or co-crystallization losses which occur due to the precipitation or co-crystallization of P.sub.2 O.sub.5 with the calcium sulfate. Lattice losses tend to increase as the amount of excess sulfate in the reactor is decreased. Second are coated rock losses which are due to an over-vigorous calcium sulfate (gypsum) crystallization. Such a crystallization causes phosphate rock particles to be covered by a gypsum layer, thus shielding the particles from further attack (reaction) by the sulfuric acid before the particles can completely react. Any phosphate rock particles that become coated with gypsum constitute a loss of P.sub.2 O.sub.5 production because the unreacted phosphate in the particle is filtered out of the phosphoric acid solution together with the other solid matter (primarily gypsum) in the tank slurry. Coated rock losses increase as the excess sulfate in the attack tank increases. Coated rock losses also may vary to some extent with the quality and with the fineness of the phosphate rock particles (rock quality is usually measured as a grade (a numerical value) in units of bone phosphate of lime (BPL)). Third are acid losses which occur due to inadequate filtration, i.e., leaving phosphoric acid solution in the filtrate. Gypsum crystal size, and thus filtration characteristics, is determined primarily by the variability in the excess sulfate. In a properly operated phosphoric acid plant, lattice losses and coated rock losses usually account for a majority of the acid losses encountered.
The recovery of P.sub.2 O.sub.5 is strongly dependent on the amount of excess sulfate in the reactor. The optimum operating point (target) for excess sulfate varies from plant to plant, and may vary with feed rate. Therefore, the optimum amount of excess sulfate for a particular operation typically is determined by periodic efficiency tests. To maximize production of P.sub.2 O.sub.5 in the reactor, the amount of excess sulfate in the reactor should be maintained as close as possible to this predetermined optimum (target) operating point. The determination of the optimum point of operation for a particular facility is well within the skill of the phosphoric acid process control art.
Another important factor that influences the efficiency of a phosphoric acid plant is operation of the filter. The reactor effluent, which comprises phosphoric acid solution and gypsum, is sent to a filtration unit wherein the solid gypsum is separated from the solution. The gypsum forms a filter cake and the filtrate comprises phosphoric acid solution. The filtrate is further treated before shipment.
The filter cake is washed with water to recover phosphorous values. Typically, a two or three-stage counter-current washing is utilized. Process water, which contains essentially no phosphorous values, is used to wash the gypsum cake a final time before the cake is removed as a by-product. In a three-stage system, the wash water, which contains some phosphoric acid, then is used in the penultimate washing, from which additional phosacid is recovered. In a third stage, the gypsum cake just separated from product acid is washed with the solution from the second washing. A portion of the solution from this third washing is introduced to the attack tank. This stream typically is known as an `acid-recycle stream`, as `return acid`, or simply as `return`.
Reactor and filtration operating parameters should be closely controlled to ensure maximum recovery of phosphoric acid from these units. Maximum recovery of phosphoric acid does not necessarily correlate with maximum concentration of phosacid in the reactor or the maximum concentration of phosacid in the product filtrate stream. Rather, the maximum recovery of product is obtained by maintaining an optimum concentration of phosacid in the reactor.
As set forth in Equation A above, water and sulfuric acid are fed to an attack tank along with phosphate ore. Typically, the ore is fed in the form of an aqueous slurry, and acid solution is recycled from the cake washings. It is typical to add a stoichiometric excess of water.
Water may be added by way of various process streams, including the feed phosphate ore ("rock") stream, the acid recycle stream from the filter, or a raw water feed stream. Water required to provide the stoichiometric quantity of water in accordance with the reaction set forth in Equation A may be added by way of any one of these streams. It is common in the trade to add the water to the attack tank by way of the return acid from the filter unit. However, an operator must take care not to add water to the filter in a quantity that will oversaturate the gypsum filter cake. As skilled practitioners recognize, it is not desirable for the moisture content of the filter cake to be excessively high because water remaining in the cake effects the water balance in the attack tank. It is desirable to maintain an overall water balance on the attack tank and filter to maintain the stoichiometric relationship set forth in Equation A in the attack tank while at the same time maintaining the efficiency of the filter unit and the moisture content of the filter cake.
Typically, in manual operation of a phosacid attack tank and filter, an operator would vary the amount of water added to the filter by visual inspection of the moisture content of the filter cake discharged and the knowledge of concentration of phosacid in the attack tank. For example, low phosacid concentration decreases cake discharge moisture content. As a consequence of the low phosacid concentration, the operator would decrease the amount of water added to the filtration unit. This decreases the rate at which water is added to the filter, and thus, less water will be added to the attack tank by way of the return acid. The lower quantity of return acid causes the phosacid concentration in the attack tank to increase.
As the phosacid concentration in the attack tank increases, the operator would add water to the filter in an attempt to bring the concentration of phosacid in the attack tank to its optimum value. However, this additional water would cause the moisture content of the gypsum filter cake to increase, and still more water would have to be added to the filter to rinse the filter cake. Under these conditions, the ability of the filter to accept more water would be marginal at best, because the gypsum crystals would be affected by the acid concentration. Also, as the concentration of phosacid in the attack tank increases, the viscosity of the reaction effluent increases, and the phosacid tends to adhere more easily to the gypsum crystals. The additional acid content of the slurry leaving the attack tank therefore causes the filterability of the gypsum filter cake to decrease and further reduces the efficiency of the filter. Thus, production rates of phosacid would decrease even though the concentration of phosacid in the attack tank was high.
Thus, keeping the phosacid concentration in the attack tank essentially constant is desirable to maximize phosphoric acid yield. Because the concentration of the phosphate ore fed to the attack tank is constantly changing, maintenance of the concentration of phosacid in the attack tank is difficult. Also, as described above, the sulfate and phosphoric acid concentrations in the attack tank influence the growth and type of calcium sulfate crystals formed in the attack tank. Thus, there is a need to control the operating conditions in the attack tank and filtration unit to ensure maximum yield of phosphoric acid.
Many problems may arise when attempting to maintain the excess sulfate in the reactor at the predetermined optimum point. If, for example, the optimum operating point was found to be zero percent excess sulfate, then the amount of sulfate desired in the reactor is that amount that allows a complete reaction of all phosphate and results in no (zero percent) excess sulfate present. The amount or flow rate of sulfuric acid necessary to achieve this optimum operating point will based on several factors including, inter alia, (1) the mass flow rate of the rock (usually measured in tons/hour), (ii) the quality or grade of the rock (CaO content), (iii) rock fineness, and (iv) reactor temperature. Therefore, the flow rate of sulfuric acid which is fed into the reactor (attack tank) required to achieve the optimum operating point (sulfuric acid demand) will vary with changes in rock mass flow rates, rock qualities, and fineness of incoming rock.
The quality or grade of the rock is determined by the calcium concentration and the type and amount of impurities in the rock. The fineness of the rock is determined by the rock itself and the process and equipment used to grind the rock. The mass flow rate of the rock, which may be considered to be the operating rate or speed of the plant, can be measured using a magnetic flow meter (if rock is fed as slurry) to determine volume flow of rock per unit time, and a nuclear density meter to measure the density of the rock slurry. Using these two values, the mass flow rate of the rock (tons/hour) may be calculated. For each rock type (including fineness, quality, etc.) there exists some demand of sulfuric acid (S) per ton of rock (R), which may be represented as the ratio S/R, which produces the desired level of excess sulfate in the reactor. This may be referred to as the desired S/R ratio.
The excess sulfate level in the reactor may be tested manually using well known laboratory methods, such as by precipitation of sulfate as barium sulfate and by measuring turbidity. The acid flow rate and the rock mass flow rate must be monitored and adjusted to maintain the excess sulfate level in the reactor equal to this predetermined level. For example, in one approach, the rock mass flow rate may be held approximately constant and the flow rate of the sulfuric acid adjusted to maintain the desired S/R ratio.
However, several additional problems exist which often hinder attempts to run the plant at this optimum operating point. First, the quality and fineness of the incoming rock are constantly changing because the quality of the rock and type of impurities in the rock vary widely among different ore sites, and even at the same site. Thus, the desired S/R ratio must frequently be updated in order to maintain the reactor at the optimum level of excess sulfate. Unexpected difficulties such as mechanical problems (i.e., broken pumps, stuck valves, and the like) often prevent maintaining the flow rates at a known constant level, and may cause the excess sulfate level in the reactor to change drastically before any malfunction is detected. Even if the sulfuric acid flow rate is properly adjusted for the particular incoming rock, the excess sulfate already present in the reactor may not be correct. Therefore, an additional adjustment in the sulfuric acid flow rate must be made to correct for any sulfate excess or deficiency already in the reactor. In addition, after an adjustment in the sulfuric acid flow rate has been made, there is an unavoidable lag time before the excess sulfate level in the reactor changes in response to the new acid flow rate.
Skilled practitioners also recognize that factors affect the ability to maintain the phosacid concentration in the attack tank at the optimum concentration. As described above, the concentration of phosphate ore in the rock fed to the attack tank constantly changes. To maintain the concentration of phosacid in the attack tank at its optimum value, the amount of water added to the attack tank must be adjusted to maintain at least the stoichiometry of Equation A above. The water addition rate must also be adjusted to compensate for differences in reaction rate caused by differences in fineness of the rock. Failure to consider fineness and its effect on reaction rate may cause the concentration of phosacid in the attack tank solution to change even though the mass flow rate of feed rock is constant.
Filterability of the reaction slurry (attack tank effluent) also affects product recovery. Because the amount of phosphate fed to the attack tank is not constant, the amount of sulfuric acid needed to effect the reaction set forth in Equation A above also changes. The amounts of product and by-product produced thus are constantly changing, and these changes affect the chemical makeup of the filter cake. Thus, rinse water may be required at the filtration unit to further rinse the filter cake and remove the phosacid entrained therein, but the additional water may not be necessary at the attack tank. Further difficulties arise when unexpected mechanical problems affect the flow rates of attack tank feed and effluent streams, and may cause the phosacid concentration in the attack tank to worsen drastically before these problems are detected.
Although plant efficiency of a phosacid plant is strongly related to the efficiency of the filtration step, control of filter operation, or of the attack tank, does not provide control of overall plant efficiency. For example, U.S. Pat. No. 3,104,946 discloses a method for controlling a wet process plant by monitoring the free sulfuric acid (sulfate) content in the attack tank by adjusting the acid-to-rock feed ratio to compensate for the sulfuric acid (sulfate) content of the recycle stream. In SU 597,632 is disclosed a method for controlling the sulfuric acid dilution process by regulating the flow rates of concentrated sulfuric acid, monohydrate, and water to maintain a constant stoichiometric ratio of diluted sulfuric acid and of the monohydrate. Plant efficiency is not considered in such control schemes.
The aims of other control schemes, such as that disclosed in U.S. Pat. No. 4,777,027, are not plant efficiency, but rather energy efficiency or reduced capital requirements. Indeed, the method disclosed in U.S. Pat. No. 4,777,027, is directed, inter alia, to production of calcium sulfate solids in a particular form (e.g., hemihydrate, dihydrate) desired for subsequent use. While maintaining conditions appropriate for a wet process producing phosphoric acid, operating conditions (in particular, reaction temperatures and flow rates of mixtures circulating around the reactors) are adjusted to produce the desired form of calcium sulfate at a concentration which achieves a desired phosphoric acid concentration.
One method said to provide efficient control over a wet process plant is disclosed in SU 1,411,276 (abstract). In accordance with that method, one obtains a material balance on the liquid streams to and from the filter and calculates the ratio of liquid to solid in the attack tank effluent. The rate at which water is added to the attack tank also is determined. The ratios of the flow rates of raw material to the flow rate of recycle and of liquid material to solid material in the attack tank are computed. The concentration of phosphoric acid in the liquid phase of the attack tank and the level in the attack tank are kept at desired operating points by adjusting the recycle rate. The ratio of liquid phase to solid phase in the attack tank also is controlled by adjusting recycle rate.
Methods of controlling reactions to produce constant production rate of a desired product, such as that disclosed in U.S. Pat. No. 3,130,187, do not maintain plant efficiency. Similarly, control schemes for reactions requiring a plurality of feed stocks, such as the scheme disclosed in U.S. Pat. No. 4,332,590, which control relative reactant concentration ratios and total feedrate to achieve a desired total production by maintaining a desired reactant concentration, do not achieve constant efficiency.
In accordance with the method of U.S. Pat. No. 4,332,590, the flow rates of all materials fed to and removed from the reactor, including any recycle streams, are manipulated to control reactor residence time. Further, the temperatures of streams fed into the reactor are controlled to achieve a desired conversion rate. The aim of this control method is to achieve a constant flow rate of a desired reactant which is introduced into the reactor in a stream in which the concentration of the desired reactant varies, so that the desired total production is achieved.
Similarly, methods for controlling operation of filters, such as that disclosed in U.S. Pat. No. 4,358,827, does not achieve constant efficiency. Rather, the object of such methods is to optimize filtration operation. Even assuming such a method, which is directed to dewatering sludge, were applicable to the phosacid/gypsum separation step, operation in accordance with the method disclosed therein would not result in a constant efficiency operation.